Kinematics, structure and relationship to metamorphism of the east‐west flow in the Sanbagawa Belt, southwest Japan

Abstract

Deformation in the Sanbagawa Belt is characterized by ductile flow in an east-west direction sub-parallel to its length. The east-west flow (D1) caused large-scale recumbent folding of the metamorphic sequence in central Shikoku, which can explain the inverted thermal structure of this region. Chemical zoning of metamorphic minerals associated with D1 microstructures also suggest that the east-west flow developed under retrograde conditions. D1 is therefore related to exhumation rather than subduction processes. A variety of kinematic indicators show that during the east-west flow, deformation was partitioned into structurally continuous domains with opposed senses of shear. This suggests that bulk deformation was not simple shear but included a component of flattening.1985; Higashino 1990) have been proposed. The Asemigawa route cross-cuts the inverted thermal structure of Central Shikoku (Fig. 2). There is good exposure along this traverse that affords good structural control across this section making it ideal for studying the structures responsible for the inverted metamorphic zonation.

Recognition of the ‘early’ Sambagawa metamorphism and a schematic cross-section of the Late-Cretaceous Sambagawa subduction zone

“…Speculated subduction-boundary geotherm at about 89 Ma Figs. 2b, 2d Ds Wallis, 1990Hara et al, 1992Wallis et al, 1992;Aoya, 2002;Mizukami and Wallis, 2005;Mori and Wallis, 2010 -3 Wallis and Ar-Ar Itaya and Takasugi, 1988;Takasu and Dallmeyer, 1990;Dallmeyer and Takasu, 1991;Itaya and Fukui, 1994 -91- Kabir and Takasu (2011) (for Seba pelite in SB), Endo and Tsuboi (2013) (for EI) and Weller et al (2015) (for SB and Kotsu). For Ota et al (2004), only P-T estimates obtained from mineral assemblages involving garnet, clinopyroxene and phengite are plotted.…”

Structural and petrological constraints on the tectonic evolution of the garnet‐lherzolite facies Higashi‐akaishi peridotite body, Sanbagawa belt, SW Japan

“…Dr is related to subduction and Ds is related to exhumation [Wallis et al, 1992;Wallis, 1998]. One of the clearest ways to distinguish between Dr and Ds is the timing of deformation with respect to growth of plagioclase [Wallis, 1990[Wallis, , 1998]: Dr structures are overgrown by plagioclase porphyroblasts (e.g., Figure 7a) and whereas Ds structures postdate the growth of plagioclase (Figures 7b and 7c).…”

“…In the Besshi unit, plagioclase porphyroblasts grew around the peak of metamorphic temperature [Otsuki and Banno, 1990;Wallis et al, 1992] and the appearance of associated minerals define the metamorphic zonation of the Besshi unit [Enami, 1982;Higashino, 1990]. If the phase of heating in Figure 7.…”

“…These metamorphic zones show an overall inverted thermal structure with a general increase in grade up structural section. However, the isograds are locally tightly folded on the kilometer-scale [e.g., Banno et al, 1986;Hara et al, 1990;Wallis et al, 1992]. The metamorphic history of the Besshi unit is characterized by a series of clockwise P-T paths involving a temperature rise after the peak of metamorphic pressure (0.6 -1.1 GPa) [Enami et al, 1994;Aoya, 2001].…”

Tectonic landform and geology of the Median Tectonic Line Fault Zone in western Shikoku, southwest Japan

“…1, 2 , 2001, , 2010, , , 1993, , 2010, 2014Nagai, 1957199919992014FT 16.5 0.7 Ma , 1996 16.8 1.1 Ma 18.4 1.5 Ma , 1999 K-Ar 16.9 0.4 Ma 15.5 0.4 Ma , 2000Ma , , 1999Kubota and Takeshita, 2008, 199950 Ma , 1999 50 Ma , Wallis et al, 2000 Chiba and Sakakibara, 2006 4.2.…”